In underground drilling applications, such as for the production of oil and gas, a wellbore or bore hole is drilled through a formation deep in the earth. Such bore holes are drilled or formed by a drill bit connected to end of a series of sections of drill pipe, so as to form an assembly commonly referred to as a “drill string”. The drill string extends from the surface to the bottom of the bore hole. As the drill bit rotates, it advances into the earth, thereby forming the bore hole. In order to lubricate the drill bit and flush cuttings from its path as it advances, a high pressure fluid, referred to as “drilling mud,” is directed through an internal passage in the drill string and out through the drill bit. The drilling mud then flows to the surface through an annular passage formed between the exterior of the drill string and the surface of the bore.
The distal or bottom end of the drill string, which includes the drill bit, is referred to as a “down hole assembly.” In addition to the drill bit, the down hole assembly often includes specialized modules or tools within the drill string that make up the electrical system for the drill string. Such modules often include sensing modules, a control module and a pulsar module. In many applications, the sensing modules provide the drill string operator with information regarding the formation as it is being drilled through, using techniques commonly referred to as “measurement while drilling”(MWD) or “logging while drilling”(LWD). For example, resistivity sensors may be used to transmit and receive high frequency signals (c.g., electromagnetic waves) that travel through the formation surrounding the sensor.
The construction of one such device is shown in U.S. Pat. No. 5,816,311 (Turner). By comparing the transmitted and received signals, information can be determined concerning the nature of the formation through which the signal has traveled, and whether the formation contains water or hydrocarbons. One such method for sensing and evaluating the characteristics of the formation adjacent to the bore hole is disclosed in U.S. Pat. No. 5,144,245 (Wisler). Other sensors are used in conjunction with magnetic resonance imaging (MRI) such as that disclosed in U.S. Pat. No. 5,280,243 (Miller). Still other sensors include gamma scintillator, which are used to determine the natural radioactivity of the formation, and nuclear detectors, which are used to determine the porosity and density of the formation.
In other applications, sensing modules are utilized to provide data concerning the direction of the drilling and can be used, for example, to control the direction of a steerable drill bit as it advances. Steering sensors may include a magnetometer to sense azimuth and an accelerometer to sense inclination. Signals from the sensor modules are typically received and processed in the control module of the down hole tool. The control module may incorporate specialized electronic components to digitize and store the sensor data.
Temperature sensitive components used for downhole operations are not limited to drilling applications but can also be utilized in wireline tools. As is well known, wireline tools include perforators, logging tools, bond evaluation tools, formation testing devices, and seismic acquisition, to name but a few.
As can be readily appreciated, such electrical systems will include many sophisticated electronic components, such as the sensors themselves, which in many cases include printed circuit boards. Additional associated components for storing and processing data in the control module may also be included on printed circuit boards. Unfortunately, many of these electronic components generate heat that are also susceptible to damage resulting from the generated heat. This is in addition to the thermal energy inherently provided by the subterranean formations surrounding the wellbore. For example, the components of a typical MWD system or a system attached to a wireline, such as but not limited to, a magnetometer, accelerometer, solenoid driver, microprocessor, power supply and gamma scintillator, may generate over 20 watts of heat. Moreover, even if the electronic component itself does not generate heat, the temperature of the formation itself typically exceeds the maximum temperature capability of the components.
Overheating frequently results in failure or reduced life expectancy for thermally exposed electronic components. For example, photo multiplier tubes, which are used in gamma scintillator and nuclear detectors for converting light energy from a scintillating crystal into electrical current, cannot operate above 175° C. Consequently, cooling of the electronic components is important. Unfortunately, cooling is made difficult by the fact that the temperature of the formation surrounding deep wells, especially geothermal wells, is typically relatively high, and may exceed 200° C.
Certain methods have been proposed for protecting such electronic components during hydrocarbon exploration and production operations within a wellbore. One such approach, which requires isolating the electronic components from the formation by incorporating them within a vacuum insulated Dewar flask, is shown in U.S. Pat. No. 4,375,157 (Boesen). The Boesen device includes thermoelectric coolers that are powered from the surface. The thermoelectric coolers transfer heat from the electronics area within the Dewar flask to the well fluid by means of a vapor phase heat transfer pipe. Such approaches are not suitable for wellbore use since the size of such configurations makes them difficult to package into a down hole assembly.
Another approach, as disclosed in U.S. Pat. No. 5,547,028 (Owens) involves placing a thermoelectric cooler adjacent to an electronic component or sensor located in a recess formed in the outer surface of a well logging tool. This approach, however, does not ensure that there will be adequate contact between the components to ensure efficient heat transfer, nor is the electronic component protected from the shock and vibration that it would experience in a drilling application.
Thus, one of the prominent design problems encountered in down hole logging tools is associated with overcoming the extreme temperatures encountered in the down hole environment. Thus, there exists a need to protect components and electronics of wellbore tools during use thereby maintaining the temperature of the components to within the safe operating level of the electronics. Various schemes have been attempted to resolve the temperature differential problem to keep the tool temperature below the maximum electronic operating temperature, but none of the known techniques have proven satisfactory.
Down hole tools are exposed to tremendous thermal strain. The down hole tool housing is in direct thermal contact with the bore hole drilling fluids and conducts heat from the bore hole drilling fluid into the down hole tool housing. Conduction of heat into the tool housing raises the ambient temperature inside of the electronics chamber. Thus, the thermal load on a non-insulated down hole tool's electronic system is enormous and can lead to electronic failure. In the event of electronic failure, down hole operations must be interrupted while the down hole tool is removed from deployment and repaired. Thus, various methods have been employed in an attempt to reduce the thermal load on all the components, including the electronics and sensors inside of the down hole tool. To reduce the thermal load, down hole tool designers have tried surrounding electronics with thermal insulators or placed the electronics in a vacuum flask. Such attempts at thermal load reduction, while partially successful, have proven problematic in part because of heat conducted from outside the electronics chamber and into the electronics flask via the feed-through wires connected to the electronics. Moreover, heat generated by the electronics trapped inside of the flask also raises the ambient operating temperature.
Typically, the electronic insulator flasks have utilized materials having a low thermal conductivity to insulate the electronics to retard heat transfer from the bore hole into the down hole tool and into the electronics chamber. Designers place insulators adjacent to the electronics to retard the increase in temperature caused by heat entering the flask. The design goal is to keep the ambient temperature inside of the electronics chamber flask below the critical temperature at which electronic failure may occur. Designers seek to keep the temperature below critical for the duration of the logging run, which is usually less than 12 hours for wireline operations.
Electronic container flasks, unfortunately, take as long to cool down as they take to heat up. Thus, once the internal flask temperature exceeds the critical temperature for the electronics, it requires many hours to cool down before an electronics flask can be used again safely. Thus, there is a need to provide an electronics and or component cooling system that actually removes heat from the flask or electronics/sensor region without requiring extremely long cool down cycles that impede down hole operations. As discussed above, electronic cooling via thermoelectric and compressor cooling systems has been considered, however, neither have proven to be viable solutions.
Thermoelectric coolers require too much external power for the small amount of cooling capacity that they provide. Moreover, few if any of the thermoelectric coolers are capable of operating at down hole temperatures. Additionally, as soon as the thermoelectric cooler system is turned off, the system becomes a heat conductor that enables heat to rapidly conduct through the thermoelectric system and flow back into the electronics chamber from the hotter regions of the down hole tool. Compressor-based cooling systems also require considerable power for the limited amount of cooling capacity they provide. Also, most compressors seals cannot operate at the high temperatures experienced down hole because they are prone to fail under the thermal strain.
Thus a need exists for shielding downhole components from the excessive thermal heating present within wellbore environments.